1. Field of the Invention
This invention relates to the art of manufacturing and, more particularly, to a method of performing measurement sampling of lots in a production process.
2. Description of Background
Conventionally, measurement sampling is determined by a lot attribute assigned at a beginning or early stage of a manufacturing process. The attribute assured that a certain percentage of work-in-progress (WIP) was measured at various inspection points before and/or after each operation of a production process. Unfortunately, the attribute did not account for performance of the operation. Towards that end, a manual query was created to suggest a sample rate based on process capacity. However, being manually run, changing/adjusting the sample rate is a labor intensive and time consuming process and thus not performed very often.
Another challenge is created by “Send Ahead” (SAHD) operations. In normal manufacturing operations, there is no need to use, for example, SAHD wafers. However, when tolerance variation in a particular process is unacceptable, SAHD is required in order to prevent scrap. SAHD is also necessary for low volume parts to speed yield learning. In either case, no SAHD lot should be skipped by a sampling plan. When relying on an attribute to determine sampling, many SAHD lots are not sampled. Without real time integrated product and process information, it is difficult, if not impossible, to establish a sampling rate that can account for a lot attribute, process performance, and SAHD lots in a manufacturing process. When relying solely on a lot attribute, or manual sampling, a lot may be sampled too often, or not often enough. Sampling, i.e., measurement is a non-value added operation and actually slows production. Thus, over sampling can be costly. However, if too few lots are sampled, defective lots can pass through production. In this case, final testing costs are increased and a company's reputation for quality is at risk.